1. Field of the Invention
The present invention relates to an original document feeding apparatus for successively feeding sheet-shaped original documents to a reading region, for use within an electrostatic copying machine or the like.
2. Description of the Related Art
Conventionally, an optical reading apparatus for exposing a surface of an original document in the shape of a slit and optically reading the surface of the original document, e.g., a transfer-type electrostatic copying machine, comprises first exposing means for fixing the original document and exposing and reading the original document while moving an optical system, and second exposing means for fixing the optical system and exposing and reading the original document while moving the original document. The second exposing means in some cases uses a circulation-type original document feeding apparatus (hereinafter "RDH") for performing automatic transportation in which sheet-shaped original documents are supplied successively one at a time to a reading region, read while exposed, and then returned to an original document position.
FIG. 58 is a cross-sectional view showing a simplified structure of a conventional RDH 201. The RDH 201 is disclosed in Japanese Unexamined Patent Publication JPA 2-175574 (1990) by the inventor of the invention. Generally, the RDH 201 comprises an original document container part 202, a first support cylinder 203, a second support cylinder 204, first original document reversing means 205 and second original document reversing means 206.
Original documents 207 stacked and stored in the original document container part 202 are transported to a transportation path 209 by a paper feeding roller 208 which forms a paper feeding mechanism, one by one successively from the uppermost original document 207a. The original document 207 inserted through an entrance 209a of the transportation path 209 is passed over a first transparent plate 212 of a first reading part 211 of the electrostatic copying machine to which the RDH 201 is mounted, by means of the first support cylinder 203 which rotates in the direction of an arrow 210. An image of one surface of the original document 207 is read by the first reading part 211.
Following this, the original document 207 is reversed by the first original document reversing means 205 which is composed of a plurality of paper feeding rollers, and direction change hooks, etc. The reversed original document 207 is passed over a second transparent plate 215 of a second reading part 214 of the electrostatic copying machine, by means of the second support cylinder 204 which rotates in the direction of an arrow 213. An image of the other surface of the original document 207 is read by the second reading part 214.
The original document 207 is further reversed by the second original document reversing means 206. The second original document reversing means 206 includes a third support cylinder 216 which rotates in the direction of an arrow 217. The original document 207 reversed by the second original document reversing means 206 is discharged from an exit 209b of the transportation path 209, and again, stored in the original document container part 202.
FIG. 59 is a cross sectional view showing a simplified structure of another conventional RDH 221. The RDH 221 is disclosed in Japanese Unexamined Patent Publication JPA 57-125950 (1982), and characterized in that transportation means 222 which serves as an original document support plate is movable in a vertical direction. First, the transportation means 222 is located below as shown by the solid line. A transportation mechanism 224 of a paper feeding mechanism 223 is lifted up to form an insertion inlet 231, an original document is inserted from the insertion inlet 231, and the original document is stacked on the transportation means 222 which is located below.
Upon stacking of the original document, the transportation means 222 moves upward as shown by the broken line, and the stacked original document is transported to a transportation path 226 by the paper feeding mechanism 223 which is composed of the transportation mechanism 224 and an air injection device 225. After passing the transportation path 226, the original document is transported to a transportation path 228 by transportation means 227, and stacked onto the transportation means 222 once again. An image of the original document is read on the transportation means 227.
After an original document passed once through the transportation path 226, the transportation means 227 and the transportation path 228 and returned, in order to store the original document between a plurality of original documents which are stacked on the transportation means 222 and the transportation means 222, a lifter 230 lifts up the original documents which are stacked on the transportation means 222. An air injecting device 229 injects air between the original documents which are stacked on the transportation means 222 and the transportation means 222. The transportation means 222 moves so that original documents on the transportation means 222 may be located close to the transportation mechanism 224 of the paper feeding mechanism 223.
The RDHs 201 and 221 are both for transporting the stored original documents one by one successively from the uppermost thereof, and storing the original document which has returned after being read by a predetermined image reading operation between the stored original documents and the original document stacking surface. To handle a large number of original documents in such RDHs, it is necessary to ensure a relatively long distance between the original document stacking surface and the paper feeding mechanism.
In the RDH 201, the paper feeding roller 208 moves in accordance with the height of the stack of the original documents 207. In this case, a paper feeding position, i.e, the position of the uppermost surface of the stack of the original documents 207 changes depending on the number of the original documents. Hence, in order to supply original documents stably, it is necessary to move the entrance 209a of the transportation path 209 in accordance with the changing paper feeding position.
On the other hand, in the RDH 221, although the transportation means 222 which serves as an original document support plate is moved, whether the transportation means 222 has moved upward to the paper feeding position without fail is not confirmed. Hence, the uppermost one of the stacked original documents is not necessarily at the paper feeding position without fail. Accordingly, stable supply of original documents is not very reliable. In addition, when the transportation means 222 is moved after an instruction to start copying is issued, it inconveniently takes a relatively long time to complete copying of the first original document. Further, the air injecting device 229 and the lifter 230 are provided in order to store a returned original document between stored original documents and the original document stacking surface. To provide such relatively large means, a large space is needed and complex control is required.